1. Field of the Invention
The present invention relates to a method of recording images on a recording material using a recording head having a plurality of recording elements as well as an apparatus therefor.
2. Description of the Related Art
Conventional ink jet or thermal printers are designed such that they record images first by performing main (horizontal) scanning by moving a recording head, in which a plurality of recording elements are arrayed, in a direction (a horizontal scanning direction) different from that in which the recording elements are arrayed and then by conveying a recording medium in a direction substantially perpendicular to the horizontal scanning direction through a distance corresponding to a recording width after the horizontal scanning is completed.
FIG. 18 illustrates how such a printer performs printing. Reference numeral 101 denotes a recording medium. Reference numeral 102 denotes a recording head in which a plurality of recording elements are arrayed. The recording head 102 is capable of recording an image consisting of 128 pixels on the recording medium 101 in one recording operation.
To form an image of one screen using the above-described type of recording head, recording of a width of 128 pixels is first performed at a pretermined density (period) by moving the recording head 102 in a direction indicated by an arrow 103 at a predetermined speed (main (horizontal) scanning) to form a band image having a width of 128 pixels.
After the recording medium 101 is moved through a distance corresponding to the width of 128 pixels in a direction indicated by an arrow 104 (sub (vertical) scanning), recording is performed again by moving the recording head 102 in a direction indicated by the arrow 103, whereby an image having a width of a total of 2 bands, i.e., 2.times.128 pixels, is formed.
An image of one screen is formed by repeating a sequence of those operations a predetermined number of times.
The above-described printer may be connected to a reader so that it can record data representing an image of an original read by a scanner. In such a case, a reading head constructed such that the reading operation thereof is associated with the operation of the above-described recording head may be used, and one screen of the image of the original may be read in the same scanning method as the scanning method of the recording head.
More specifically, such a reading head has a sensor capable of reading 128 pixels in one reading operation, and the reading direction of the reading head corresponds to the recording direction of the recording head. Accordingly, the image read by the reading head in one reading operation is recorded by the recording head in one recording operation.
FIG. 19 is a block diagram of a copying machine which contains the above-described type of reading and recording mechanisms.
Reference numeral 110 denotes an image reading portion (scanner) having the above-described type of reading mechanism. Reference numeral 111 denotes an image processing portion for performing predetermined processes on the image data read by the scanner 110. Reference numeral 113 denotes an image recording portion (printer) having the above-described type of recording mechanism. Reference numeral 112 denotes a head shading portion (HS) for correcting irregularities of the recording head using the input image data.
The recording head has recording elements respectively corresponding to 128 pixels. The recording elements perform recording utilizing, for example, thermal energy. If the recording head is of a thermal transfer type, the recording elements are heating elements. If the recording head is of an ink jet recording type, the recording elements are ink nozzles.
All of these plurality of recording elements do not reproduce the same density when they receive the same amount of thermal energy, i.e., there are variations in the density provided by the recording elements.
The conventional recording head adopts the binary reproduction method. In this method, a reproduced virtual density is adjusted by correcting the average densities near a single recording element.
FIG. 20 shows the structure of the HS portion 112.
Reference numeral 116 denotes a HS-RAM formed of a RAM for the convenience of exchange of the recording head. The HS-RAM stores variations in the recording elements of the recording head in a numerical form. Reference numeral 117 denotes a HS-ROM which outputs corrected image data from both the output of the HS-RAM 116 and the image data.
FIG. 21 shows the structure of the printer portion 113.
Reference numeral 120 denotes a binarizing portion for binarizing input image data and for outputting the binarized image data. The binarizing portion 120 is capable of performing adequate binarization, such as the dither method or the error diffusion method, on natural images, such as character images or photographs. Reference numeral 121 denotes a buffer memory for temporarily storing the binarized image data obtained by the binarizing portion and for sending the stored image data to the recording head 122.
In the copying machine arranged in the manner described above, image data of 128 pixels read by the scanner portion 110 is output to the image processing portion 111 in sequence. The image processing portion 111 performs predetermined processes on the image data and sends the processed data to the head shading portion 112. The image data on which variations of the recording head have been corrected by the head shading portion 112 is binarized by the binarizing portion 120. The binarized data is stored in the buffer memory 121. When the image data of 128 pixels are stored in the buffer memory 121, the recording head 122 starts recording.
If the image data processing speed is sufficiently faster than the horizontal scanning speed of the recording head and if the the response speed of the recording head can be made to correspond to the image data processing speed, the image data may be sequentially recorded at the processing speed of the image data without the image data of 128 pixels being stored in the buffer memory 121.
To obtain a reduced image of the original, only m pixels corresponding to 64 pixels (half of the recording head) of a reduced image are made effective in one reading operation, as shown in FIG. 22. First, recording of the former 64 pixels of the recording head is performed in the first horizontal scanning operation (a), and then the scanner portion alone is moved for vertical scanning without the printer portion being moved for vertical scanning. Thereafter, recording of the latter 64 pixels is performed in the second horizontal scanning operation, and then both the scanner portion and the printer portion are moved for vertical scanning (b).
If the reduction rate is 50%, the number of pixels which are made effective in the first horizontal scanning operation is obtained as follows: EQU m.times.50%=64 pixels, m=128
If the reduction rate is 64%, EQU m.times.64%=64 pixels, m=100
The amount of movement for vertical scanning of the scanner portion is m pixels.
In the printer portion, after recording of 64 pixels is performed using the upper half recording elements of the recording head, recording of 64 pixels is performed using the lower half recording elements of the recording head without the recording medium being conveyed. Thereafter, the recording medium is conveyed. Thus, the amount of movement of the recording head for vertical scanning is always 128 pixels. In the head shading portion 112, image data is corrected according to nozzle number.
However, in the above-described conventional reduction recording method, since the recording elements of 64 pixels out of 128 pixels are used alternately, some elements of the recording head are used for recording while the other elements are not used in one horizontal scanning operation.
Accordingly, heat for recording is transferred from the recording elements which are used for recording to the recording elements which are not used near the center of 128 pixels, affecting these recording elements which are not used. Thus, these recording elements may have thermal energy more than that required when they are used next time. Further, the heat of the recording elements which are used is transferred to the recording elements which are not used, and the recording elements cannot perform recording at the normal level of thermal energy.
Consequently, a stripe-shaped density irregularity may be generated at every 128 pixels.
Hence, a HS portion exclusively used for removal of such a density irregularity may be provided. However, the number of such HS portions must be the same as the number of portions into which the recording elements are divided for reduction, thus increasing production cost or the size of the copying machine.
Further, in the case of, for example, down to 50% of reduction, if the width of reading is 128 pixels, the reduced image has a width which is less than 128 pixels and not less than 64 pixels. At that time, the effective image has a width corresponding to 64 pixels, and the remaining image is discarded and read again in a subsequent band.
Thus, a total number of scanning operations required to scan one page in reduction recording increases from the number of scanning operations required for a life-size recording. If the reduction rate is, for example, 99%, the total number of scanning operations is about twice that for life size recording, thus increasing the copying time by a factor of about 2.